The present invention relates to an adhesive tape which is low in cost and the properties of which such as unwindability, adhesiveness, trimmed clearance, printability and transparency are improved without spoiling hand lateral cutting characteristics and, more particularly, it relates to an adhesive tape having improved longitudinal tearing strength.
In conventional adhesive tape, as shown in FIG. 9, a molten synthetic resin material 101 is extruded into a sheet by an extruding machine 102, and the sheet is passed through a rubber roll 103 and a drawing roll 104 to form a smooth surface on one side and an uneven surface on the other side and then processed by corona discharge so as to facilitate the coating of an adhesive, thereafter a base film 107 is produced by passing through an electrode 105 and a processing roll 106.
As shown in FIG. 10, the base film 107 passing through the electrode 105 and the processing roll 106 contacts the former at the smooth surface 108 and contacts the latter at the uneven surface 109. The base film 107 thus prepared is coated with an adhesive on the smooth surface 108 to produce the adhesive tape.
Prior art disclosing an adhesive tape, in which an adhesive layer is provided on the smooth surface and the other surface is formed to be uneven, includes Japanese Publication patent No. 13306/1975 entitled "Adhesive Tape" and Japanese Publication Utility Model Patent No. 47907/1976 entitled "Plastic Adhesive Tape".
In the prior art an adhesive tape which is formed in a body by providing a smooth adhesive layer on the smooth surface and forming the other surface to be uneven, exhibits the following disadvantages.
(1) As shown in FIG. 10, in order to improve adhesion of the adhesive on the smooth surface 108 of the base film 107, the smooth surface 108 is processed by corona discharge by discharging from the electrode 105. But the corona discharge is struck through the uneven surface 109. That is, the uneven surface 109 is also partly processed by the corona discharge and susceptible to stick to the adhesives coated on the smooth surface 108 at that partly processed portion, so that the rolled adhesive tape is difficult to unwind and blocking may occur.
(2) When using the adhesive tape, the adhesive layer surface is deformed unevenly by the uneven surface on the rear side, and the adhesive power is inevitably weakened since the adhesive area is reduced.
(3) When utilizing the adhesive tape as a trimming tape for obtaining a distinct coating line, as shown in FIG. 11, a coating material permeates into recesses 112 of the adhesive layer 111 of the base film 107, and the coating line L cannot be finished distinctly as shown in FIG. 12. That is, the trimmed clearance of the coating is poor.
(4) Since the adhesive tape has an uneven rear surface, printing, stamping or writing on the rear surface cannot be made clearly. That is, the printability properties of the tape are poor.
(5) Since the rear surface of the adhesive tape is formed unevenly, light is reflected irregularly to cause poor transparency.
As a result of research by the inventors carried out to solve such properties as unwindability, adhesiveness, trimmed clearance, printability and transparency, the inventors have developed an adhesive tape comprising a polyolefin resin base film formed with a smooth surface on one side and an uneven surface on the other side, and an adhesive layer coated with an adhesive on said uneven surface.
With regard to the unwindability property of an adhesive tape, this characteristic is practically very important among the various characteristics exhibited by the adhesive tape.
In order to facilitate the unwindability of a tape and to increase the anchoring force of an adhesive, the surface wetting tension of the adhesive coated surface must be larger than that of the rear side.
Therefore, there are such means as coating an anchor coating agent on the adhesive coated surface or corona discharge processing in order to increase the difference of surface wetting tension, and further a method of coating a stripping agent on the rear side. Among them the corona discharge processing is the best, because it is inexpensive and suitable for mass production.
In economical corona discharge processing, though the processing roll and an electrode are maintained at a given interval and high voltage is applied to initiate the corona discharge while passing a plastic film therebetween, marks, stains and dust stuck to the processing roll may frequently cause corona discharge between the rear side of the base film and the processing roll. Since the rear side is also subjected to the corona discharge processing, the surface wetting tension of the rear surface may increase. That is, if the plastic film is not tightly contacted to the processing roll, the corona discharge is struck through it.
In the prior art, however, since the smooth surface must be processed by the corona discharge and the rear side is uneven, the rear side of the base film is not contacted to the processing roll tightly, thus the corona discharge is struck through and the surface wetting tension is increased. When the adhesive tape is produced by such a base film, its practical use may be hindered by poor unwindability and blocking which occurs.